In many modern gasoline-fueled engines, the fuel is injected sequentially and directly into each cylinder of a multi-cylinder engine. There the fuel is mixed with at least a stoichiometric amount of air, spark-ignited, and burned to produce the powered stroke of the piston. Combusted gas is exhausted from each engine cylinder by reciprocating piston action and combined in one or more exhaust manifolds. The exhaust enters an exhaust duct in which it is treated by flowing through one or more catalytic converters before the exhaust is discharged to the atmosphere.
Many gasoline-fueled engines are operated by cycling the air-to-fuel ratio closely around the stoichiometric mass ratio. In this mode of engine operation, the exhaust is typically directed through the many longitudinal channels (e.g., 400 per inch2 of inlet face) of an extruded cylindrical ceramic monolith. The walls of such flow channels are provided with a washcoat of platinum group metal (PGM) particles supported on alumina particles. Such a catalytic converter is called a 3-way converter because it cleans the exhaust gas by promoting the oxidation of both unburned hydrocarbons and carbon monoxide and, concomitantly, the reduction of nitrogen oxides (collectively NOx) to nitrogen. Other gasoline-fueled engines may be operated at a higher-than-stoichiometric air-to-fuel ratio. This mode of engine operation is called “lean burn” and it produces more oxygen in the exhaust which makes it more difficult to reduce NOx. A lean burn engine may use a monolithic converter coated with an oxidation-only PGM formulation to oxidize unburned hydrocarbons and carbon monoxide. The exhaust may then be further treated to oxidize NO to NO2 and then subjected to selective catalytic reduction (SCR) of NO2 to N2.
The exhaust gas from fuel injected, gasoline-fueled, spark-ignited engines is also found to contain small carbon-containing, generally spherical particles. Generally, the particulate material averages about 70 nanometers in diameter and is typically less than about 200 nanometers in diameter or greatest dimension. These particles may agglomerate into small clusters. Attention may now be given to the management of such particulate matter from spark-ignited gasoline engines.
Diesel engines produce larger volumes of particulate matter per unit of operating time than gasoline-fueled engines. The particulate matter from such compression-ignition engines is accumulated as a soot cake on a porous filter. The accumulating soot cake thereafter serves as a very effective filter medium. But, from time to time during engine operation, the diesel soot layer becomes an intolerable resistance to exhaust gas flow, impeding engine operation, and has to be burned off the underlying filter body. Diversion of the vehicle's diesel fuel is required for burning-off the diesel soot. This diversion lowers the fuel efficiency of the vehicle and the environmental performance of its exhaust system. The practice of accumulating soot as a filter medium and periodically burning the exhaust soot with vehicle fuel is not considered to be a suitable option for removal of particulate matter from a gasoline-fueled engine.
It is an object of this invention to provide a practice for the filtration and concomitant passive oxidation of such particles in the exhaust system of a gasoline-fueled vehicle. It is a further object of this invention to take advantage of the relatively high temperature of the exhaust leaving a gasoline-fueled engine to obtain the passive oxidation of filtered particulates. Thus, it is a further object to accomplish the management of such small carbon particles with minimal effect on the oxidation or reduction of the gaseous exhaust contaminants or the fuel economy of the engine.